In the transmitter portion of a transceiver used for communicating digital data, a baseband information signal is typically scrambled, encoded, and interleaved before being modulated and then up-converted to a radio frequency, and amplified, for radio (wireless) transmission. The receiver portion of the transceiver performs the reverse processes including down-converting a received RF signal to baseband.
As is well known by persons skilled in the art, in a traditional heterodyne receiver the up and down conversion from baseband to RF and from RF to baseband, respectively, is done in two stages to introduce an intermediary frequency (IF). In the up conversion process (converting from baseband to RF), a first stage uses an IF oscillator signal and a second stage uses an RF oscillator signal. This two-stage process is used because the side band signals (referred to as the image side band and the desired side band) that would result from a single stage up or down conversion would require the use of a high Q, off-chip filter to extract the information signal (representing a relatively high cost solution).
For the two-stage up conversion process in the transmitter, the baseband information signal is first converted to an IF band information signal (this first stage being the up-conversion IF stage) by multiplying the baseband signal by an IF oscillator signal using a mixer. This produces two side band signals centred around the IF. To extract the IF band information signal a narrow band filter is used (i.e. this filter being sufficiently narrow to exclude any signals in the undesired image band or IF carrier feed-through signal). In the second stage (being the up-conversion RF stage) the filtered IF band information signal is again converted to a higher frequency band, this time to an RF band information signal, by multiplying the IF band information signal by an RF oscillator signal using a mixer. This produces two side band signals centred around the RF but, because the frequency spread between them is at least twice the IF, the desired band signal can be filtered out using a relatively simple (low Q) band pass filter.
In a multi-channel transceiver several different adjacent band RF information signals are received. At each down conversion stage of the receiver, this results in the presence of undesired signals in the image band for a desired information signal and these undesired signals must be excluded. For the first down conversion stage (referred to herein as the down conversion RF stage), a mixer multiplies the RF information signal by an RF oscillator signal and this results in adjacent band RF signals being translated to adjacent band IF signals. To extract the desired IF band information signal from these other adjacent band signals, a narrow band filter is used. In the second stage (referred to herein as the down conversion IF stage) the extracted IF band information signal is again down-converted by multiplying the IF band information signal by an IF oscillator signal using a mixer. This produces the desired base band information signal.
In the foregoing traditional heterodyne down conversion architecture it is necessary to provide a sufficiently low IF frequency to permit sufficient filtering to occur to achieve high levels of rejection of the adjacent band signals, and in particular, the image side band signals.
More recently, with the development of an on-chip double quadrature mixer (also known as a complex mixer), a new receiver architecture is being proposed which would use the good tracking ability of a double-quadrature mixer to achieve substantial rejection (e.g. 40 dB) of the undesired image band signal. It is proposed in the industry that such use of a double-quadrature mixer will be used in conjunction with use of the narrow band IF filter of the traditional architecture to complement the results of filtering. As for the traditional architecture, this new architecture which makes use of a double-quadrature mixer topology is also limited to a relatively low IF because the known double-quadrature mixers are non-operative at high RF frequencies. It is known that a realizable double-quadrature mixer is presently limited to maximum operating frequency of about 500 MHz.
Since the usage of a narrow band filter at the IF frequency band fixes the IF frequency, the known heterodyne receiver architectures require a frequency synthesizer that is able to “tune” all RF channel frequencies to a single IF frequency. For example, for a receiver designed to receive RF information signals in accordance with the 802.11a 5 GHz wireless standard, the receiver must be able to tune to each of the 8 different RF channels defined by this standard. In addition, it must provide a constant value IF oscillator signal to down-convert the IF band information signal to base band. This need for both a tunable and a fixed frequency signal for the heterodyne down-conversion process has been fulfilled in known receivers by a relatively complex frequency synthesizer design incorporating at least two feedback loops with individual VCO's (voltage controlled oscillators).
Accordingly, there is a need for an improved design for transceiver up and down converters. In addition, there is a need for a transceiver up and down converter using a simplified frequency synthesizer.